Some two-phase fluids, such as fluids obtained from water dominated geothermal sources such as occur naturally in many areas of the world, contain a considerable amount of heat that can be utilized for power production using a Rankine cycle power plant. Because of the corrosive properties of such geothermal fluid, and engineering difficulties in designing heat engines to operate efficiently with a two-phase working fluid, it is conventional to separate the water and steam at the wellhead, and to utilize the two fluids in separate power plants. The steam may be used, either directly in a conventional steam turbine, or indirectly as the heat source for an organic working fluid Rankine cycle power plant such as shown in U.S. Pat. No. 4,542,625, the disclosure which is hereby incorporated by reference. Such an organic, Rankine cycle power plant comprises a vaporizer containing an organic fluid, such as a Freon, hydrocarbon, etc., which is vaporized by the application of the geothermal steam to the vaporizer. The resultant heat-depleted steam (condensate) at the outlet of the vaporizer is then disposed of, possibly by injection into what is termed a rejection well. Vaporized working fluid produced by the vaporizer is applied to a specially designed turbine which converts some of the heat in the working fluid to useful work and produces heat-depleted working fluid that is supplied to either a water, or an air cooled condenser wherein the heat-depleted working fluid is condensed into a liquid condensate that is returned to the vaporizer.
To increase the utilization of the heat contained in geothermal fluid, it is also conventional to use the liquid separated from the steam for generating power. To this end, it is conventional to apply the hot geothermal water to another organic Rankine cycle power plant such as shown in U.S. Pat. No. 4,578,953, the disclosure of which is hereby incorporated by reference. The '953 patent discloses a cascade arrangement of vaporizers, each associated with its own turbine, and arranged so that the hot geothermal water passes serially from vaporizer to vaporizer. Improved thermodynamic efficiency is achieved by employing a preheater for each vaporizer and applying the heat-depleted geothermal water from the last stage of the cascade to all the preheaters in parallel, thereby preheating the working fluid in each stage before the working fluid enters the vaporizer of the stage.
In some situations, it is impractical to provide separate power plants, one utilizing geothermal steam, and the other utilizing geothermal water. Furthermore, the thermodynamic efficiency of a power plant operating on geothermal water may be too low to warrant the capital cost of the equipment.
It is therefore an object of the present invention to provide a new and improved method of and means for increasing the thermodynamic efficiency of a power plant operating with a two-phase fluid such as fluid obtained from a water dominated geothermal source.